Chap 20: Magnetic Properties
a) Transmission electron micrograph
showing the microstructure of the
perpendicular magnetic recording
medium used in hard-disk drives.
b) Magnetic storage hard disks used in
laptop (left) and desktop (right)
computers.
c) Inside of a hard disk drive.
d) Laptop computer
Chapter 20 -
Generation of a Magnetic Field -Vacuum
• Created by current through a coil:
B
I
N = total number of turns
 = length of each turn (m)
I = current (ampere)
B = magnetic field (tesla)
Chapter 20 - 2
Generation of a Magnetic Field -within a Solid Material
• A magnetic field is induced in the material
B
B = Magnetic field (tesla)
inside the material
current I

• Relative permeability (dimensionless) r 
0
Chapter 20 - 3
Origins of Magnetic Moments
• Magnetic moments arise from electron motions and the
spins on electrons.
magnetic moments
electron
electron
spin
nucleus
electron orbital
motion
Adapted from Fig. 20.4,
Callister & Rethwisch 8e.
electron
spin
• Net atomic magnetic moment:
-- sum of moments from all electrons.
• Four types of response...
Chapter 20 - 4
B (tesla)
Types of Magnetism
(3) ferromagnetic e.g. Fe3O4, NiFe2O4
(4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd
( cm as large as 106 !)
(2) paramagnetic ( cm ~ 10-4)
e.g., Al, Cr, Mo, Na, Ti, Zr
vacuum (cm = 0)
(1) diamagnetic (cm ~ -10-5)
e.g., Al2O3, Cu, Au, Si, Ag, Zn
H (ampere-turns/m)
Plot adapted from Fig. 20.6, Callister & Rethwisch 8e.
Values and materials from Table 20.2 and discussion in
Section 20.4, Callister & Rethwisch 8e.
Chapter 20 - 5
Magnetic Responses for 4 Types
none
opposing
Adapted from Fig.
20.5(a), Callister &
Rethwisch 8e.
(2) paramagnetic
random
aligned
Adapted from Fig.
20.5(b), Callister &
Rethwisch 8e.
(3) ferromagnetic
(4) ferrimagnetic
aligned
Applied
Magnetic Field (H)
aligned
No Applied
Magnetic Field (H = 0)
Adapted from Fig.
20.7, Callister &
Rethwisch 8e.
(1) diamagnetic
Chapter 20 - 6
Influence of Temperature on
Magnetic Behavior
With increasing temperature, the saturation magnetization diminishes
gradually and then abruptly drops to zero at Curie Temperature, Tc.
Chapter 20 - 7
Magnetic Domains
Chapter 20 - 8
Domains in Ferromagnetic &
Ferrimagnetic Materials
• As the applied field (H) increases the magnetic domains
change shape and size by movement of domain boundaries.
B sat
H
Magnetic
induction (B)
H
H
H
H
0
• “Domains” with
aligned magnetic
moment grow at
expense of poorly
aligned ones!
Adapted from Fig. 20.13,
Callister & Rethwisch
8e. (Fig. 20.13 adapted
from O.H. Wyatt and D.
Dew-Hughes, Metals,
Ceramics, and
Polymers, Cambridge
University Press, 1974.)
Applied Magnetic Field (H)
H=0
Chapter 20 - 9
Hysteresis and Permanent
Magnetization
• The magnetic hysteresis phenomenon
B
Stage 3. Remove H, alignment
remains! => permanent magnet!
Stage 2. Apply H,
align domains
H
Stage 4. Coercivity, HC
Negative H needed to
demagnitize!
Stage 5. Apply -H,
align domains
Adapted from Fig. 20.14,
Callister & Rethwisch 8e.
Stage 1. Initial (unmagnetized state)
Stage 6. Close the
hysteresis loop
Chapter 20 - 10
Magnetic Anisotropy
Easy magnetization direction:
Ni- [111], Fe- [100], Co- [0001].
Hard magnetization direction:
Ni- [100], Fe- [111],
Co-
Chapter 20 - 11
Hard and Soft Magnetic Materials
B
-- large coercivities
-- used for permanent magnets
-- add particles/voids to
inhibit domain wall motion
-- example: tungsten steel -Hc = 5900 amp-turn/m)
Soft
Hard magnetic materials:
H
Soft magnetic materials:
-- small coercivities
-- used for electric motors
-- example: commercial iron 99.95 Fe
Adapted from Fig. 20.19, Callister & Rethwisch
8e. (Fig. 20.19 from K.M. Ralls, T.H. Courtney,
and J. Wulff, Introduction to Materials Science
and Engineering, John Wiley and Sons, Inc.,
1976.)
Chapter 20 - 12
Iron-Silicon Alloy (97 wt% Fe – 3 wt% Si)
in Transformer Cores
Transformer cores require soft magnetic materials, which are easily
magnetized and de-magnetized, and have high electrical resistivity.
Energy losses in transformers could be minimized if their cores were
fabricated such that the easy magnetization direction is parallel to the
direction of the applied magnetic field.
Chapter 20 -
Magnetic Storage
• Digitized data in the form of electrical signals are transferred to
and recorded digitally on a magnetic medium (tape or disk)
• This transference is accomplished by a recording system that
consists of a read/write head
-- “write” or record data by applying a
magnetic field that aligns domains
in small regions of the recording
medium
-- “read” or retrieve data from
medium by sensing changes
in magnetization
Fig. 20.23, Callister &
Rethwisch 8e.
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Chapter 20 - 14
Magnetic Storage Media Types
-- CoCr alloy grains (darker regions)
separated by oxide grain boundary
segregant layer (lighter regions)
-- Magnetization direction of each
grain is perpendicular to plane of
disk
80 nm
• Hard disk drives (granular/perpendicular media):
Fig. 20.25, Callister
& Rethwisch 8e.
(Fig. 20.25 from
Seagate Recording
Media)
• Recording tape (particulate media):
~ 500 nm
~ 500 nm
Fig. 20.24, Callister
& Rethwisch 8e.
(Fig. 20.24
courtesy Fuji Film
Inc., Recording
Media Division)
-- Acicular (needle-shaped)
ferromagnetic metal alloy
particles
-- Tabular (plate-shaped)
ferrimagnetic barium-ferrite
Chapter 20 - 15
particles
Superconductivity
Found in 26 metals and hundreds of alloys & compounds
Mercury
Copper
(normal)
4.2 K
Fig. 20.26, Callister &
Rethwisch 8e.
• TC = critical temperature
= temperature below which material is superconductive
Chapter 20 - 16
Critical Properties of
Superconductive Materials
TC = critical temperature - if T > TC not superconducting
JC = critical current density - if J > JC not superconducting
HC = critical magnetic field - if H > HC not superconducting
 T 2 
HC (T )  HC (0)1  2 
 TC 

Fig. 20.27, Callister &
Rethwisch 8e.
Chapter 20 - 17
Meissner Effect
• Superconductors expel magnetic fields
normal
superconductor
Fig. 20.28, Callister &
Rethwisch 8e.
• This is why a superconductor will float above a
magnet
Chapter 20 - 18
Advances in Superconductivity
• Research in superconductive materials was stagnant
for many years.
– Everyone assumed TC,max was about 23 K
– Many theories said it was impossible to increase
TC beyond this value
• 1987- new materials were discovered with TC > 30 K
– ceramics of form Ba1-x Kx BiO3-y
– Started enormous race
• Y Ba2Cu3O7-x
TC = 90 K
• Tl2Ba2Ca2Cu3Ox TC = 122 K
• difficult to make since oxidation state is very important
• The major problem is that these ceramic materials
are inherently brittle.
Chapter 20 - 19
Summary
• A magnetic field is produced when a current flows
through a wire coil.
• Magnetic induction (B):
-- an internal magnetic field is induced in a material that is
situated within an external magnetic field (H).
-- magnetic moments result from electron interactions with
the applied magnetic field
• Types of material responses to magnetic fields are:
-- ferrimagnetic and ferromagnetic (large magnetic susceptibilities)
-- paramagnetic (small and positive magnetic susceptibilities)
-- diamagnetic (small and negative magnetic susceptibilities)
• Types of ferrimagnetic and ferromagnetic materials:
-- Hard: large coercivities
-- Soft: small coercivities
• Magnetic storage media:
-- particulate barium-ferrite in polymeric film (tape)
-- thin film Co-Cr alloy (hard drive)
Chapter 20 - 20